(1) Field of the Invention
This invention relates to a reformer for hydrogen generation, suitably used in industries and vehicles.
(2) Description of the Related Art
In recent years, production of electricity without causing environmental pollution has drawn attention and an interest in fuel cell has increased. Fuel cell has various advantages such as high efficiency in power generation, formation of small amount of carbon dioxide (CO2) and substantially no formation of harmful substances such as carbon monoxide (CO), nitrogen oxides (NOx) or the like. Therefore, researches and developments on use of fuel cell in on-site type power generator or automobile have been under way recently. In generating electricity using a fuel cell, high-purity hydrogen is required. This hydrogen is produced by using, as a starting material, a hydrocarbon (e.g., butane or propane), an alcohol (e.g., methanol), CO or the like and conducting a catalytic reaction.
The main reaction in the above hydrogen production is steam reforming which takes place in the presence of steam and a catalyst. Since the steam reforming is generally an endothermic reaction although it differs depending upon the starting material used, it is important to heat the catalyst to a desired temperature uniformly. Decrease in reaction temperature invites formation of coke and resultant deactivation of catalyst; therefore, great care is necessary in industrial designing of the reactor.
Further, since the above steam reforming has a low reaction speed unlike combustion reaction, a relatively large catalyst volume is required in treating a given amount of a starting material. Meanwhile, the catalyst functions at high temperature. Hence, a long time is taken to warm up the catalyst. Thus, there have been problems when the steam reforming is utilized in an on-site generator or an automobile where quick hydrogen generation is required.
In conventional catalytic processes for hydrogen production by steam reforming, the catalyst used has generally been heated externally. When a starting material is passed over a fixed catalyst bed and a relatively large reaction tube is used, it is difficult to transfer heat to the center of the catalyst bed and there has been used a complicated mechanism that a tubular reactor is heated by the use of a heating medium such as metal bath, combustion waste gas or the like.
In other conventional catalytic process for hydrogen production by steam reforming, the heating of the catalyst used has been conducted by introducing a combustion waste gas (generated in gas-phase reaction or catalytic combustion) into the reaction tube and heating the catalyst with the heat of the waste gas. This process is not preferred because it increases the flow amount of fluid, reducing the activity of intended reaction and generating more CO2 by combustion.
In the gas produced by the steam reforming, hydrogen has no sufficient purity to be used in a fuel cell and CO has a deactivating effect on the Pt-based electrode used in the fuel cell. Therefore, a CO shift reaction (an aqueous conversion reaction) and a CO selective oxidation reaction are conducted to increase the purity of hydrogen. However, there are many technical problems as to the way in which the catalysts used therein are heated so as to function or the way in which the reactions are allowed to proceed stably.
As still another process for generating hydrogen from a hydrocarbon or the like, there is a process which comprises generating hydrogen and CO by a partial oxidation reaction of a hydrocarbon in place of the above-mentioned steam reforming and then conducting the above-mentioned CO shift reaction and CO selective oxidation reaction to obtain hydrogen. In this process, the partial oxidation reaction of the first step is an exothermic reaction and is substantially free from the problem of heat supply; however, since the reaction temperature is generally higher than that of the steam reforming, technical problems remain unsolved as to how the catalyst temperature is maintained and how high-purity hydrogen is generated in a short time when the process is utilized in an on-site generator or an automobile. As still another process for generating hydrogen from a hydrocarbon or the like, there is a process using a decomposition reaction. A specific example of the decomposition reaction is a decomposition reaction for generating hydrogen from methanol. This reaction is an endothermic reaction similar to the steam reforming, and hence there are the similar problems as mentioned above.
Also in industries where hydrogen is consumed in a large amount, such as ammonia synthesis, hydrogenation, hydrodesulfurization or the like, there are many technical problems to be improved in areas such as reaction efficiency, low operational energy, period of reactor start-up and conversion of the starting material.
The inventors of the present invention have proposed a reformer of new configuration, as disclosed in Japanese Patent Application No. 9-296004, in an attempt to solve the above problems. This reactor disposed in the flow path of a reactant fluid, which comprises:
a catalyst unit capable of generating hydrogen from a reactant fluid containing an organic compound or carbon monoxide, by catalysis; and
an electrically heatable heater unit, wherein honeycomb structures are suitably used both for the catalyst and heater units.
The reactor can generate, in a short time, high-purity hydrogen for a fuel cell utilized in an industrial unit or automobile. However, the inventors of the present invention have found, after extensive studies to still improve the reactor, that there are room for improvements in, e.g., contact efficiency between the catalyst unit and reactant fluid, and heat-exchanging efficiency of the heater units, when two or more units are installed.
The present invention, is achieved based on the above recognition, aims at providing a reformer, disposed in the flow path of a reactant fluid, which comprises a catalyst unit capable of generating hydrogen from a reactant fluid containing an organic compound or carbon monoxide, by catalysis, and an electrically heatable heater unit, wherein improvements are made on the above-mentioned reactor in, e.g., contact efficiency between the catalyst unit and reactant fluid, and heat-exchanging efficiency of the heater units, when two or more units are installed.
According to the present invention, there is provided, as a first invention,
a reformer disposed in the flow path of a reactant fluid, which comprises:
an electrically heatable heater unit of honeycomb structure, in the upstream of the flow path of a reactant fluid, and
at least one catalyst unit of honeycomb structure capable of generating hydrogen from a reactant fluid containing an organic compound or carbon monoxide, by catalysis, in the downstream of the above heater unit, wherein the heater units and at least one of the catalyst units satisfy the following relationship:
Cell density of the heater unitxe2x89xa6Cell density of the catalyst unit.
Furthermore, according to the present invention, there is provided, as a second invention,
a reformer disposed in the flow path of a reactant fluid, which comprises:
two or more electrically heatable heater units of honeycomb structure; and
at least one catalyst unit of honeycomb structure capable of generating hydrogen from a reactant fluid containing an organic compound or carbon monoxide, by catalysis, at least at one position between the above heater units,
wherein, at least one of the heater units upstream of the catalyst unit (upstream-side heater unit) and at least one of the heater units downstream of the catalyst unit (downstream-side heater unit) satisfy the following relationship:
Cell density of the upstream-side heater unitxe2x89xa7Cell density of the downstream-side heater unit.
Furthermore, according to the present invention, there is provided, as a third invention,
a reformer disposed in the flow path of a reactant fluid, which comprises:
two or more electrically heatable heater units of honeycomb structure, adjacent to each other, installed in the direction of fluid flow; and
a catalyst unit of honeycomb structure capable of generating hydrogen from a reactant fluid containing an organic compound or carbon monoxide, by catalysis, wherein, at least two adjacent heater units satisfy the following relationship:
Cell density of the upstream-side heater unitxe2x89xa6Cell density of the downstream-side heater unit.
Still more, in the present invention, the term xe2x80x9choneycomb structurexe2x80x9d means a structure having a number of through-holes (cells) separated by a partition from each other. Also in the present invention, xe2x80x9ca catalyst unit of honeycomb structure capable of generating hydrogen from a reactant fluid containing an organic compound or carbon monoxide, by catalysisxe2x80x9d includes a catalyst unit of honeycomb structure which shows a catalytic reaction of increasing purity of hydrogen generated from a reactant fluid containing an organic compound or carbon monoxide, by catalysis, e.g., CO shift reaction and selective oxidation of CO.